Recently, communication systems using electromagnetic waves in the microwave range, such as automobile phones, portable telephones, satellite broadcasting systems or the like have been developed so that miniaturization of their terminal apparatus have been required. In order to miniaturize the terminal apparatus, each part of apparatus needs to be miniaturized. In such apparatus, the dielectric ceramic is placed in a frequency stabilizing device of various filters and oscillators. When the resonant mode is the same, the size of these resonant devices is inversely proportional to the square root of the dielectric constant (.epsilon..sub.r) of the dielectric materials to be used. Consequently, in order to make miniature resonant devices, materials having a high dielectric constant are needed. Other required properties of the dielectric ceramic include: having a low loss in a microwave range, in other words, having a high Q value; and having a small temperature coefficient at a resonant frequency (.tau..sub.r). As a material having a high dielectric constant, a (Pb, Ca) ZrO.sub.3 system is disclosed in Japanese Laid-Open Patent No. (Tokkai-Hei) 4-65021. This system has a high dielectric constant (.epsilon..sub.r) of more than 100, a high Q value of approximately 800 at 2-4 GHz and a small temperature coefficient at a resonant frequency.
Meanwhile, attempts have been made to miniaturize resonance devices and to improve the function by making a multilayer structure of conductor and dielectric ceramic. Since a conductor to be used in such a high-frequency range as microwave is required to have a high conductivity, Cu, Au, Ag or an alloy of them need to be used. In the case of making the dielectric ceramic multilayer structure, dielectric ceramic is required to be burned at the same time when the conductive metal is burned. The heat of the sintering operation acts on the conductive metal as well as the ceramic, so that the sintering conditions must be mild enough that the metal does not melt or oxidize. In other words, the dielectric ceramic is required to be densely sintered at temperatures below the melting points of the conductive metals to be used (the melting point of Cu is 1083.degree. C., that of Au is 1063.degree. C. , that of Ag is 961.degree. C.). Furthermore, if the Cu is used for the electrode, the dielectric ceramic is required to be burnt at low oxygen partial pressure. As a microwave dielectric ceramic that can be sintered at low temperature, a Bi.sub.2 O.sub.3 -CaO-Nb.sub.2 O.sub.5 system is disclosed in Japanese Laid-Open Patent No. (Tokkai-Hei) 5-225826.
Although the Bi.sub.2 O.sub.3 -CaO-Nb.sub.2 O.sub.5 system ceramic can be sintered at a low temperature around 1000.degree. C., Bi.sub.2 O.sub.3 of the main component evaporates during burning. Thus, the dielectric property is unstable with respect to the burning temperature. In general, when the resonant device is further miniaturized, the Q value drops. Therefore, suitable materials having a high Q value are still needed.